1. Field of the Invention
The present invention relates to printing apparatus and methods, and more particularly to lithographic printing plate constructions that may be imaged on- or off-press.
2. Description of the Related Art
Traditional techniques of introducing a printed image onto a recording material include letterpress, flexographic and gravure printing, and offset lithography. All of these printing methods require a printing member, usually loaded onto or integral with a plate cylinder of a rotary press for efficiency, to transfer ink in the pattern of the image. In letterpress and flexographic printing, the image pattern is represented on the printing member in the form of raised areas that accept ink and transfer it onto the recording medium by impression; flexographic systems, which utilize elastomeric surfaces, have received more widespread acceptance due to the broad variety of compatible substrates and the ability to run with fluid inks. Gravure printing cylinders, in contrast to raised-surface systems, contain series of wells or indentations that accept ink for deposit onto the recording medium; excess ink must be removed from the cylinder by a doctor blade or similar device prior to contact between the cylinder and the recording medium.
In the case of offset lithography, the image is present on a plate or mat as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry printing system, the plate is simply inked and the image transferred onto a recording material; the plate first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to or in conjunction with inking. The ink-rejecting fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
The plates for an offset press are usually produced photographically. To prepare a wet plate using a typical negative-working subtractive process, the original document is photographed to produce a photographic negative. This negative is placed on an aluminum plate having a water-receptive, anodized (textured) surface coated with a presensitized photopolymer. Upon exposure to actinic radiation through the negative, the areas of the coating that received radiation (corresponding to the dark or printed areas of the original) cure to a durable oleophilic state. The plate is then subjected to a developing process that removes the uncured areas of the coating (i.e., those which did not receive radiation, corresponding to the non-image or background areas of the original), exposing the hydrophilic surface of the aluminum plate. Conventional wet plates also typically contain primer layers, which provide better anchorage of the photopolymer to the aluminum substrate.
In a positive-working process, the areas of the photosensitive coating that received radiation become labile and are removed by development; it is the unexposed areas that persist and carry ink. Photoexposure processes are well-understood and common in the industry.
Rendering a layer of aluminum, which is hydrophilic but fragile in an unstructured or polished state, sufficiently durable to repeatedly accept fountain solution in a printing environment requires special treatment. Any number of electrochemical techniques, in some cases assisted by the use of fine abrasives to further roughen the surface, may be employed for this purpose. For example, electrograining involves immersion of two opposed aluminum plates (or one plate and a suitable counterelectrode) in an electrolytic cell and passing alternating current between them. The result of this process is a finely pitted surface topography that readily adsorbs water. See, e.g., U.S. Pat. No. 4,087,341.
A structured or grained surface can also be produced by controlled oxidation, a process commonly called "anodizing." The anodized aluminum plate consists of an unmodified base layer and a porous, "anodic" aluminum oxide coating thereover; this coating readily accepts water. However, without further treatment, the oxide coating would lose wettability due to further chemical reaction. Anodized plates are, therefore, typically exposed to a silicate solution or other suitable (e.g., phosphate) reagent that stabilizes the hydrophilic character of the plate surface. In the case of silicate treatment, the surface may assume the properties of a molecular sieve with a high affinity for molecules of a definite size and shape--including, most importantly, water molecules. The treated surface also promotes adhesion to an overlying photopolymer layer. Anodizing and silicate treatment processes are described in U.S. Pat. Nos. 3,181,461 and 3,902,976.
Textured chromium surfaces also exhibit substantial hydrophilic character, and can be used in lieu of aluminum in wet-running lithographic plates. Such surfaces can be produced by, for example, electrodeposition, as described in U.S. Pat. No. 4,596,760. As used herein, the term "textured" refers to any modification to the surface topography of a metal plate that results in enhancement of hydrophilic character.
Although printing plates containing conventional textured substrates exhibit adequate durability in commercial printing contexts, the structured nature of these surfaces renders them vulnerable to eventual wear and degradation. As these surfaces lose structure, hydrophilicity suffers and printing quality deteriorates. This is a largely unavoidable result of systems that rely on structured surfaces yet subject them to the direct action of considerable mechanical pressures and various chemical reagents.